In no particular order:
James Buchanan - Did nothing to prevent the civil war, and left Lincoln to clean up his mess. He was also a huge advocate of slavery, which makes him a detestable prick by default.
Warren G. Harding - Without a doubt the most corrupt president in history; his scandals make Nixon look like a saint.
Herbert Hoover - Stood by and let the Great Depression spiral out of control. Historians also widely agree that most other politicians found him impossible to work with.
George Bush Jr. - He signed the Patriot Act into law (read: took away the fucking rights that everybody in this country is so proud of), initiated two wars based on faulty intelligence, created a multitrillion dollar fiscal deficit, managed to turn every country in the world except for Israel against us...I don't give a shit about how history remembers him; I lived through his administration and I can tell you that he was an awful president.
Andrew Johnson - A white supremacist shithead who fucked up reconstruction after the civil war, and took a firm side with the ex-confederates.
Carter, Nixon, Reagan, Grant, Bush Sr., Pierce, Coolidge, Truman, and several others also rank high on my shit list. USA has had far more bad leaders than good.
EDIT: Hey, somebody make a thread opposite to this since I'm too lazy to.